1. Field of the Invention
The present invention relates to an optical space transmission module for transmitting an optical signal through an external space as a medium.
2. Description of the Background Art
As a speed-up mode of wireless transmission, an optical space transmission technique has recently been attracting attention. This technique uses light waves instead of radio waves. Light waves provide high security because of their nature of straight travel and light blocking, in addition to high-speed performance using their natural wide band. As a light source used in the optical space transmission, a light emitting diode (LED) or a semiconductor laser diode (LD) is used. The LD, which enables high-speed modulation, is more advantageous for improving transmission speed.
The LD, however, has a light source size which is smaller than that of the LED, and its output light has higher coherency. Thus, when the output light of the LD is emitted directly to an external space and accidentally gets into an eye, an image with high energy density is projected on a retina, and there is a danger that the retina is damaged. In order to obtain with the LD a safety level equivalent to that of the LED, light output intensity has to be lowered, or the size of a virtual secondary light source has to be enlarged with the coherency of the output light lowered using an optical component such as a diffuser panel or the like. In the latter case, the size of the image projected on the retina depends on the secondary light source size. For obtaining larger light output with safety of an optical transmitter maintained, the above secondary light source size may be enlarged. As such, an optical system which maintains safety of a laser, for example, there is an optical transmitter which is disclosed in Japanese Laid-Open Patent Publication No. 2004-165957 (hereinafter, referred to as Patent Document 1). FIG. 10 is a view showing an exemplary configuration of the conventional optical transmitter disclosed in the Patent Document 1.
As shown in FIG. 10, the conventional optical transmitter comprises a laser 900, a condenser lens 910, a lens 920, a reflection type diffusion section 921, a reflection section 922, an opening section 923, and a light receiving section 930. The condenser lens 910 converges a laser beam A outputted from the laser 900, and emits the converged laser beam A to the reflection type diffusion section 921 through the opening section 923. The reflection type diffusion section 921 reflects and converts the emitted laser beam A into diffused light B. The reflection section 922 reflects the diffused light B. The lens 920 distributes the diffused light B reflected by the reflection section 922 in one direction, and outputs the distributed light as a transmission signal C. The light receiving section 930 receives a signal beam outputted from an optical transmitter facing thereto. In the conventional optical transmitter, the coherency of the laser beam A is disturbed by the reflection type diffusion section 921, and a virtual secondary light source having a Lambertian distribution is formed on the reflection type diffusion section 921. Therefore, safety is enhanced more than the case of using a laser alone.
In the conventional optical transmitter, however, the laser beam A is emitted directly to the reflection type diffusion section 921. Thus, the reflected diffused light B is partially returned to the laser 900, and this may cause the laser 900 to operate unstably. In addition, since the conventional optical transmitter uses a reflection mechanism, its thickness can be reduced but the diameter of the reflection section 922 is large. This increases the entire occupying area of the optical transmitter including the light receiving section 930.